The present invention relates to high energy density electrochemical cells and more particularly to nonaqueous electrolytes for rechargeable, high energy density electrochemical cells having anode-cathode couples of alkali metals and transition metal trichalcogenides.
A recently developed rechargeable, high energy density electrochemical cell consisting of an electronegative material as the anode-active material, a transition metal trichalcogenide as the cathode-active material, and a nonaqueous electrolyte is described in U.S. Pat. No. 3,864,167. For example, a typical cell consists of a lithium anode, a titanium trisulfide cathode and a nonaqueous electrolyte consisting of lithium salts dissolved in organic solvents such as propylene carbonate. This type of cell has very recently been improved by the inclusion of dopants such as ethers in the electrolyte to enhance cell life, as described in U.S. Pat. No. 3,928,067.
An important feature of these cells is their ability to be discharged and charged. Theoretically, cycling by discharging and charging should be possible indefinitely, but in practice indefinite cycling is not realized. Dendritic growth on the anode during charging and degradation of the cathode material are usually the limiting factors in the amount of cycling a cell can be subjected to; but the electrolyte, particularly a nonaqueous electrolyte, can at times be the limiting factor. The effects of a particular solvent on the electrochemical performance of a cell cannot be determined on theoretical grounds, but must be ascertained empirically. A particular organic electrolyte might be highly effective with a given anode-cathode couple but be ineffective for another couple, either because it is not inert to the second couple or because it reacts with itself under the conditions present during cycling. Furthermore, an organic electrolyte might be perfectly suited for use in a primary cell and not usable for secondary cells.
Industrial use of any high energy density cell is dependent upon a number of factors including initial cost and service life. Wide industrial use of any electrochemical cell is dependent upon extending the service life of all components of the cell, including the electrolyte. Thus, improved electrochemical cell systems containing various organic electrolytes have been developed, as taught, for example, in U.S. Pat. No. 3,947,289. However, it is believed that the electrochemical cells of the present invention containing alkali metal anodes, trichalcogenide cathode-active materials, and the described electrolytes have not been heretofore disclosed.